The invention relates to carpet having a low density frothed polyurethane backing, and to methods of making such carpet.
Carpets backed with an attached polyurethane cushion have been commercially available for many years. Methods for making those carpets are described, for example, in U.S. Pat. Nos. 4,296,159, 4,336,089, 4,405,393, 4,483,894, 4,853,054, 4,853,280, 5,104,693, 5,646,195, 6,372,810 and 6,790,872.
The attached cushion is prepared by applying a polyurethane-forming composition to the back of the carpet and permitting the composition to cure in place. A cellular structure is created by incorporating a gas into the stabilized polyurethane-forming composition. There are two main ways of doing this. One way is by including a chemical blowing agent (typically water) in the composition. The chemical blowing agent reacts as the polyurethane-forming composition cures, generating a gas (typically carbon dioxide) that imparts a cellular structure to the foam. The second way, which has found favor in commercial processes, is to froth the polyurethane-forming composition by whipping air or another gas into it.
The use of chemical blowing agents is generally disfavored because these systems are difficult to process in attached cushion applications. Although chemical blowing agents are widely used to make a variety of polyurethane foams, in most of the cases the formulation is one which should react very quickly. The speed of the reaction helps the polymer to quickly build molecular weight and become viscous. The highly reactive and viscous mixture is able to capture the expansion gases as they are generated and thus prevents the foaming mixture from collapsing or even expanding at all. These fast-curing systems cannot be used easily in carpet-backing applications, because the polyurethane-forming formulation must be mixed, dispensed onto the carpet, spread over the carpet back and gauged. It is very difficult to accomplish all of these processing steps before a chemically blown polyurethane formulation cures to an extent that it becomes unworkable. Thus, for example, U.S. Pat. No. 4,405,393 to Tillotson describes a chemically blown polyurethane system for forming attached cushions onto carpet. Tillotson describes the need to carefully control his process in order to avoid forming pockets of gas that escape from the expanding polyurethane formulation. In addition, these processes require a fast-traversing mix head and/or messy air knife to spread the mixture onto the substrate. In practice, these problems of process control have severely limited the use of chemically blown systems in commercial scale carpet backing operations.
For this reason, commercial processes almost uniformly use a frothed polyurethane system. These systems are formulated so that they react slowly when first mixed. The slow initial cure allows enough time for the formulation to be mixed, dispensed, applied uniformly to the back of the carpet and gauged. The gauged foam formulation is thereafter cured relatively quickly by exposing it to an elevated temperature.
The problem with frothed systems is that it is difficult to produce a density of below 10-12 pounds/cubic foot in the cured polyurethane foam. Lower foam densities are desired in certain carpet products, to obtain a softer feel, because performance demands are less stringent, or simply to reduce costs. The inability to produce lower density frothed polyurethane carpet backings has limited the range of carpet products that have been produced with attached polyurethane cushions.
In some cases, water has been incorporated into frothed polyurethane carpet backing formulations. Examples of this practice are described, for example, in U.S. Pat. Nos. 4,853,280 and 6,372,810. In the process described in U.S. Pat. No. 4,853,280, an attached polyurethane cushion is prepared from a frothed system, to which about 0.1% by weight of water (˜0.2% based on the unfilled system) is added. The density of the resulting cushion is reported as 11 pounds/cubic foot. In U.S. Pat. No. 6,372,810, about 10-fold higher levels of water are incorporated into a frothed foam formulation. However, the density of the resulting attached cushion is still reported as over 12 pounds/cubic foot. The additional water does not result in a further reduction of froth density, even taking into account the higher filler levels reported the examples of U.S. Pat. No. 6,372,810.
Thus, a practical, easily controllable method by which a lower density, attached polyurethane cushion can be applied to carpet or other textiles is an unmet need in this industry.